Recently, various types of digital data reproduction apparatuses have been suggested for reproducing a recording medium, such as an optical disk, a magneto-optical disk or a magnetic tape, on which digital data such as video, sound or text has been recorded.
An exemplary conventional digital data reproduction apparatus will now be described with reference to the drawings.
FIG. 23 is a block diagram of a conventional digital data reproduction apparatus outputting reproduced data.
In FIG. 23, reference numeral 101 denotes an optical disk on which digital data such as video, sound or text data has been recorded. Reference numeral 102 denotes a pick up which is used as a detector for reading data on the optical disk 101, and includes an actuator provided therein. Reference numeral 103 denotes waveform equalization means for performing a waveform equalization operation on a signal DRF, which has been obtained by reading data from the optical disk 101 with the pick up 102, so as to produce an equalized reproduced signal RRF. Reference numeral 104 denotes error signal production means for performing an arithmetic operation on the signal DRF obtained by the pick up 102 so as to produce an error signal ERS which is required for a control operation, and for performing a fine adjustment of the error signals ERS using a jitter detection signal JTR from jitter detection means 108. Reference numeral 105 denotes control means for controlling focusing, tracking, traversing, or the like, of the pick up 102 using the error signals ERS.
Reference numeral 106 denotes reproduced signal digitization means for digitizing the reproduced signal RRF, whose waveform has been equalized by the waveform equalization means 103, by comparing the signal RRF with a reference digitization level. The reproduced signal RRF is digitized by the reproduced signal digitization means 106 and output as reproduced data DDS. Reference numeral 107 denotes synchronization means for producing a clock TSJ whose phase is synchronized with the leading and trailing edges of the digitized signal which has been digitized by the reproduced signal digitization means 106. Reference numeral 108 denotes jitter detection means for detecting a jitter component contained in the reproduced signal DDS which has been digitized by the reproduced signal digitization means, so as to output the level thereof as the jitter detection signal JTR. The jitter detection method includes the method disclosed in Japanese Laid-Open Patent Publication No. 8-87756.
Now, a correlation representing the variation in the jitter level JTR with respect to the reproduced signal digitization level DSL will be described, along with the operating principle of the synchronization means 107, with reference to FIGS. 24 and 2.
FIG. 24 is a principle diagram showing a characteristic correlation between the reproduced signal digitization level DSL and the jitter level JTR, wherein the waveform on the upper portion of the figure is the reproduced signal RRF which is input to the reproduced signal digitization means 106, and the waveforms on the lower side of the figure are each an output TSJ to be output from the synchronization means 107 which is obtained by digitizing the reproduced signal RRF with one of the respective levels A, B and C of the reproduced signal digitization level DSL.
FIG. 2 is a characteristic diagram showing a reproduced signal digitization level DSL-to-jitter level JTR characteristic, wherein the horizontal axis represents the reproduced signal digitization level DSL and the vertical axis represents the output JTR of the jitter detection means 108.
In FIG. 24, when the digitization level DSL is set to level A, the clock TSJ produced by the synchronization means 107 becomes a spike-like clock as shown beside the label "For level A" in the lower portion of the figure. Then, DSL=level A holds in the characteristic diagram of FIG. 2. When the digitization level DSL is set to level B of FIG. 23, the clock TSJ produced by the synchronization means 107 becomes a pulse-like clock as shown beside the label "For level B" in the lower portion of the figure which has a time width TAB in proportion to the difference between levels A and B. Then, DSL=level B holds in the characteristic diagram of FIG. 2, and the jitter JTR is increased from that for level A. Similarly, when the digitization level DSL is set to level C of FIG. 24, the clock TSJ produced by the synchronization means 107 becomes a pulse-like clock as shown beside the label "For level C" in the lower portion of the figure which has a time width in proportion to the difference between levels A and C. Then, DSL=level C holds in the characteristic diagram of FIG. 2, and the level of jitter is increased from that for level A as in the case of level B. Thus, the level of jitter increases/decreases depending upon changes in the reproduced signal digitization level DSL.
There is an apparatus for adaptively setting the reproduced signal digitization level using as a reference the aperture ratio of the eye pattern of the reproduced signal for the purpose of optimizing the reproduced signal digitization level of such a digital data reproduction apparatus. This apparatus determines the level using a ROM table which has been previously set according to the aperture ratio of the eye pattern of the reproduced signal, and performs a digitization operation on the reproduced signal using the determined level (Japanese Laid-Open Patent Publication No. 5-6619, etc.).
However, the conventional digital data reproduction apparatuses have the following problems:
(1) The reproduced signal digitization level cannot be corrected until a signal is actually reproduced; PA1 (2) The reproduced signal digitization level may not be adjusted so as to minimize the jitter which may cause an error in reading the reproduced signal; PA1 (3) The jitter detection sensitivity in detecting a jitter component contained in a reproduced signal varies among various circuits; and PA1 (4) The individual difference becomes large among circuits for setting the cut-off frequency for waveform equalization of the reproduced signal.
The present invention has been made to solve the above-described problems and has an objective of providing a digital data reproduction apparatus and a method for correcting a reproduced signal digitization level in which the quality of the reproduced signal from the recording medium is kept high while ensuring the reproduction stability.